Make a penny disappear with water

We may earn a commission from links on this page.

Wanna watch a pyrex dish and some water make a penny disappear? How about a little more water (in an unusual place) making it come back? Take a look at a confusing physics demonstration that you can do at home, whenever you want to frustrate your guests.

Light knows that the vast majority of the population is entirely dependent on it to get around. Sometimes it displays great noblesse oblige, giving us rainbows and sparkling diamonds. Most of the time, though, it takes advantage of our dependence by playing tricks on us. One of these tricks is shown in the below video.

In this video, a penny placed under a pyrex beaker disappears the moment water is poured in beaker. When looked at from the top of the beaker, the coin is visible. When looked at from the side, it's gone. If the penny is placed inside the beaker, and water is poured in, it's perfectly visible again. What's more, if the penny is gotten wet and put under the beaker full of water again, it's still visible, despite being invisible when the coin is dry. What's going on?


Massive differences in the index of refraction are what's going on. A material's index of refraction is a measure of how fast light moves through it. Light only moves at 'light speed' through a vacuum. Anywhere else it is effectively slowed down. If it were slowed down the same amount in all materials, we wouldn't see half the amount of optic phenomena we do. When light moves from one level of refraction to another, it bends. The relatively large difference in the index of refraction for water and air (1.33 versus 1.0), is why our fingers look distorted, or broken off from the hand, when we put them in water. Pyrex, the beaker that the penny is under, has an index of refraction of 1.47. That's much closer to water than it is to air.

So when the penny is at the bottom of the beaker, there's only one sharp turn that the light from the penny has to make, from the pyrex dish to the air. The water-to-pyrex transition is comparatively mild, with little bending. The penny is distorted, but it's visible. When the wet penny is beneath the dish, but under another layer of water, the light also only has one sharp turn — back into the air at the end of its journey. Before that it only travels through water and pyrex, which have similar indices of refraction, and so it isn't bent much.


When the penny 'disappears,' though, it is taking two sharp turns, the massive turns between the pyrex and the air both at the bottom of the beaker and at the side. And, because of the way light bends, both turns are in the same direction, away from the eye of the viewer. Imagine the beaker full of water like an immense piece of rectangular carpeting on the concrete floor, and the light like a person on roller skates. (No, seriously, this will help.) The viewer is on the right side of this carpet. In order for them to see the penny, just under the bottom of the carpet, the light has to get from the penny to them. That means it has to head up and right.


The bend that light takes when it hits a new material a little bit like the turn a person on roller skates takes when they hit a new material. When you go from a piece of smooth concrete to carpeting, if the left skate hits the carpet first, as it would when the skater is trying to head to the right, the skate is slowed down and you turn left as you cross over onto the carpeting. When you roll off the carpet onto concrete again, if your right skate hits the easier material first, as it would in this case, it will zoom ahead and you'll turn left again. Two sharp left turns will turn the light from the bottom away from the viewer to the side of the beaker. The refractive index simply bends the light off course, and the penny disappears.

So why can we see the penny under an empty beaker? Because, in this case, the course of the light is self-correcting. It hits the beaker and turns sharply left. Then it hits the air inside the beaker, left side first, and the left 'skate' zooms ahead, turning it right. Then it hits the beaker again, right 'skate' first, and it's slowed down and turned to the right again. Then it hits the air, right 'skate' first again, and is turned back to the left. Left turn, right turn, right turn, left turn - they add up to a straightish course. The light from the bottom of the beaker hits the viewer's eye, and you can see it. Until you pour water in it.


Image: Rusted Strings

Second Image: Physics Central
Via Phyics Central.